The use of transient electromagnetic techniques to inspect containers such as pipelines is disclosed in Spies U.S. Pat. No. 4,843,320 and in Lara U.S. Pat. No. 4,843,319, which patents are owned by the assignee of the present invention. The '320 patent is directed to a transient electromagnetic method for detecting corrosion on conductive objects. Often, such objects are wrapped in insulation to either prevent undue heat loss or to protect personnel from hazards. The transient electromagnetic method of the '320 patent can penetrate the insulation to probe the conductive wall underneath. This layer of insulation may vary in thickness from one location to another along the container wall. My '319 patent provides a method for compensating for variations in the thickness of insulation.
The transient electromagnetic method of the '320 and '319 patents requires placing a transmitting antenna and a receiving antenna in proximity to the object. In the case of an insulated pipeline, the antennas are placed on top of the insulation. The transmitting antenna then induces a current into the pipeline wall, which current decays rapidly. The decay of the induced current is detected by the receiving antenna. By analyzing the induced current decay, a measurement of wall thickness can be obtained. Corrosion acts to reduce wall thickness, thus any reduction in wall thickness can be determined from the TEM measurement.
As shown in the '320 and '319 patents, the transient electromagnetic method is unconcerned with the speed of data acquisition. However, there exists many situations where speed of the inspection process is of primary concern. This is particularly true in downhole tubing, heat exchanger tubing and buried fluid transmission pipelines where, because of the situs of the tubing or pipeline, the antennas must be located interiorly of the tubing or pipeline. As a result of using interiorly located antennas, downtime is incurred. Lengthy downtimes produced by the use of stationary antennas are costly and are therefore avoided in practice.
Prior art technologies are not very satisfactory for inspecting downhole tubing and the like. Ultrasonic methods have narrow resolutions that add to inspection time. Also, the ultrasonic transducers are affected by fluid coupling from the fluid inside of the tubing or pipeline. Flux leakage methods have no such coupling problem, but there is a problem with interpreting the data. The signals are strongly affected by sharp edges, and make interpretation difficult.
Transient electromagnetic methods do not suffer these disadvantages. Therefore, it is desirable to increase the speed of transient electromagnetic inspections of an object. With such an inspection system, larger areas could be inspected within shorter periods of time. Furthermore, with such an inspection system, the antennas could be placed inside of pipelines and not impede the flow of fluid. Thus, the pipelines undergoing inspection could remain in use.